The present invention relates to a screen, e.g. a screen with a screen cylinder for screening pulp in pulp and paper industry, a screen cylinder per se, a method of its manufacture, and a method of utilization of a screen cylinder of the invention.
Screening of pulp in the pulp and paper industry is generally performed by using screen cylinders with openings therethrough for separating the accepts and rejects portions of the pulp. In many screen cylinders grooves are provided in the inlet and outlet side surfaces of the screen plate, for adjusting the flow characteristics and improving flow capacity of the screen. Screening openings, i.e. sizing slots, are machined or otherwise made by other methods from either the grooved side or the contour (inlet) side of the screen plate. Two to twelve groups or rows of axially extending grooves are arranged one after the other along the axis of the cylinder. A cylindrical land portion is formed between each neighboring row of grooves.
Rings have in most cases been secured on the outflow side of the screen cylinder in order to compensate for the weaker construction of the grooved cylinder compared to the strength of a blank cylinder. The rings ensure stiffness, rigidity and structural strength of the cylinder. Especially in pressurized screens, rings are needed to ensure rigidity. Rings have been secured to the screen cylinder by welding them circumferentially about the cylinder.
The rings have typically been fastened by welding them to the cylindrical land portions formed between the rows of grooves. The welds have been made by conventional welding techniques to form a protruding welded seam on each side of the ring. It would be very difficult to fasten a ring onto grooved portions of a screen cylinder, i.e. perpendicularly to the grooves on the ridges formed between neighboring parallel grooves, with such conventional welding methods and the results would not be satisfactory. In addition, thick welds (typically 3-6 mm) and especially if applied to screen cylinder surface with grooves have a tendency to cause under cuts in the narrow ridges on the groove side, causing stress-risers with potential for development of fatigue cracks. Thick welded seams would block a substantial number of screening openings in the grooves and thereby decrease the effective open area of the screen and consequently the screening throughput or flow capacity. Thick weldings could also distort the land portions between parallel grooves and slots, which would have a detrimental effect on screening. While the construction of U.S. Pat. No. 5,200,072 (the disclosure of which is hereby incorporated by reference herein) addresses this problem for screen cylinders with long grooves, the above related difficulties with conventional welding and the detrimental effects of the thick welds are still significant for screen cylinders with most conventional length slots and grooves, and are still greater in screens with unusually small slot widths.
In conventional screening cylinders, only a limited percentage of the cylinder area has screening openings, slots or the like. This limits the flow through the screen, i.e. the flow capacity. It is not simply a matter of increasing the number of apertures through the screen plate to compensate for such reduced numbers of screening openings or reduced open area, as predetermined circumferential spacings between openings must usually be maintained. Also structural considerations limit the open area. Further, the aforementioned rings providing structural strength limit the open area of the screen, as the rings have required a considerable land area to be welded to. The land areas which have to be provided for the reinforcement rings at certain axial distances considerably restrict the length of grooves and screening slots.
It is not possible with conventional means to increase the distance between reinforcement rings and land areas significantly from what is conventionally used and thereby increase the length of slots. Slot length is conventionally between 35-65 mm, typically 50 mm. Longer distances between rings would lead to decreased stability and e.g. to slot width continuously changing due to pressure variations induced by foils or rotors used for back pulsing accept suspension. Rotor power applied when inducing positive and negative pulses on the cylinder can exceed 100 kW/m.sup.2 and thereby cause high flow acceleration and rapid changes of pressure affecting the surface of the screen cylinder and slots. Undesirable movement of land portions, "land bridges", between slots, due to the above mentioned rotor action causes fatigue.
Slotted screen cylinders have, especially when manufactured with conventional milling tools, a tendency to create sensitive stress-risers at the four corners of the slots. A fast running rotor (25-30 m/s), and its mostly negative pulses creating elements, causes highly aggressive hydrodynamic conditions forcing the cylinder surface to oscillate in a"mode. The amplitude and frequency of the oscillations can cause the development of fatigue cracks initiating from the earlier mentioned stress risers.
A safe fatigue-cracking problem avoiding screen cylinder design would therefore have to be reinforced with frequent support rings and relatively short grooves/slots for greater stability. Increasing the number of rings or decreasing the length of slots would however decrease the open area, i.e. the flow capacity, of the screen, which of course is undesirable. On the contrary there has long been a need to increase the flow capacity of screens.
There is a general goal of decreasing slot width in screen cylinders, in order to achieve a cleaner accepts flow. This has also been possible to achieve, due to improved flow conditions around slot openings, developed during the last decade. Smaller slot widths lead, however, to decreased open area in the screen. Screens with 0.35 mm slots may have had an open area of about 6%, whereas comparable screens with only 0.1 mm slots have an open area of about 1%-1.5%. This decrease of open area and slot width leads to increased resistance to flow, and accordingly decreased flow capacity. A change from 0.2 mm slots to 0.1 mm slots generally leads to a decrease in open area of about 50%, and a decrease in flow capacity of about 70%.
There has long been a need for screen cylinder structures with increased open slot area, and the above-described changes in slot width further increases this need. To address this need, it has been suggested in U.S. Pat. No. 3,631,981 that contoured reinforcement rings could be welded (e.g. by a single weld) on solid circumferential land areas on the screen cylinder, the rings being contoured around the slots to provide a slight increase in the length of the groove or slot in either end close to the circumferential land area. However, this gives a very small increase in open area, and the attachment mechanism has proven to cause mechanical strength problems with rings cracking in the weld and then falling down, particularly with smaller slots and relatively high consistencies where high kW rotors are used.
Therefore there is a need to provide an improved grooved type screen cylinder with increased open area yet secure mechanical strength properties compared to conventionally fabricated screen cylinders. There is a need to provide a screen with a grooved screen cylinder in which open area and thereby flow capacity can be increased compared to conventional grooved screen cylinders of its kind without decreased cleanliness, and to provide an improved method of manufacturing grooved screen cylinders.
The present invention provides a screen with a grooved screen cylinder, for use in pulp and paper industry, which has substantially increased open area, increased efficiency, increased flow capacity and/or increased strength characteristics compared to prior grooved screen cylinders of its kind. The screen cylinder according to the invention is also simple to manufacture compared to prior art methods of forming such screens.
According to one aspect of the present invention a screen cylinder for screening suspensions to provide an accepts portion and a rejects portion is provided. The screen cylinder comprises the following components: A cylinder having an outer surface, an inner surface, a central axis, and an effective axial length, one of the inner and outer surfaces comprising an outlet side of the cylinder, and the other of the inner and outer surfaces comprising an inlet side of the cylinder. A plurality of grooves substantially parallel to the central axis formed in the outlet surface, disposed in a plurality of rows with a plurality of parallel grooves disposed, in sequence, in each row. A slot provided in at least some of the grooves, defining a through-extending flow path of a predetermined size between the inlet and outlet surfaces. At least some of the plurality of rows separated from each other by a first substantially cylindrical land area. The grooves within a row are separated from each other at the outlet surface by a second land area much smaller than the first land area. At least one first reinforcing ring is fastened to a the first land area for providing stability to the cylinder. And, at least one second reinforcing ring is permanently fastened to at least a majority of the second land areas in at least one row of grooves to provide additional stability to the cylinder without significantly adversely affecting the flow of accepts through the slots.
In many screen cylinders a slot will be provided in all (or substantially all) of the grooves. However cylinders may be constructed in which other openings (e.g. round holes) may be provided in at least some of the grooves.
Depending upon the actual height of the screen cylinder, it may comprise 1-20, typically 4-10, preferably 5-8, axially disposed rows of grooves with a cylindrical land portion between each two neighboring rows of grooves. A second reinforcing ring fastened to a groove area is, according to a preferred embodiment of the present invention, fastened by welding [e.g. continuous laser welding or by spot welding] to the second land areas, between neighboring grooves. Such a ring may, according to another embodiment of the present invention, be fastened by continuous electron beam welding, or spot welded by electron beam, to the second land portions, or by direct resistance welding, fusing each land area between adjacent relief grooves to the reinforcing ring.
Typically each second reinforcing ring is welded to substantially all of the second land areas in one row of grooves by a first weld, each of the first welds having a width of about 1-3 mm. Preferably each of the first welds has a width at least about 75% of the width of a second land area on which it is formed, and a length of at least about 50% of the width the second reinforcing ring thereat. Using the reinforcing construction according to the present invention a screen cylinder may be constructed wherein the sum of the axial lengths of slots in a column of grooves extending axially in a straight line along the cylinder divided by the effective length of the cylinder is between about 0.65-0.9 (preferably 0.8 to 0.9) which compares to prior art ratios of about 0.45-0.55; that means that about 65-90% (preferably about 80-90%) of the screen length is grooved, providing much open area.
In one embodiment of the invention, at least one of the second reinforcing rings (typically at least two rings are provided for a conventional cylinder where the plurality of rows of grooves comprise 4-10 circumferential rows of grooves) comprises a composite ring formed of axially spaced first and second components welded to each other, or a composite ring formed of radially spaced first and second components connected together.
The screen cylinder described above is best suited for screening pulps in the lower consistency range, e.g. between about 0.3-1.5%, and high flow volumes where highly aggressive (high power) rotors actions are not required. However where the consistencies are between about 1.5-6.0%, or otherwise where aggressive rotors are used (that is where the power consumption is above about 30 kW/m.sup.2 of cylinder surface area), instead of--or preferably in addition to--the rings described above a metal (e.g. steel) backing support cylinder with large square punched openings can also be provided, e.g. attached to the rings, e.g. by welding.
In some screen cylinders (having staggered slot rows), the circumferential solid land areas are interrupted by grooves (with slots) which bridge them, and are staggered between the normal rows of grooves and slots. In such cylinders the first and second rings used are essentially the same as in the conventional constructions, and have substantially the same spacings between them, the first rings merely have the welds thereof interrupted by the staggered, bridging, grooves.
According to another aspect of the present invention a method of manufacturing a screen cylinder is provided comprising the following steps: (a) Constructing a cylinder having an outer surface, an inner surface, a central axis, and an effective axial length, one of the inner and outer surfaces comprising an outlet side of the cylinder, and the other of the inner and outer surfaces comprising an inlet side of the cylinder, by: (a1) forming in the outlet surface a plurality of grooves substantially parallel to the central axis, disposed in a plurality of rows with a plurality of parallel grooves disposed, in sequence, in each row; and (a2) forming a slot provided in at least some of the grooves, each slot defining a through-extending flow path of a predetermined size between the inlet and outlet surfaces; the forming steps (a1) and (a2) being practiced so that at least some of the plurality of rows are separated from each other by a first substantially cylindrical land area, and so that the grooves within a row are separated from each other at the outlet surface by a second land area much smaller than the first land area. (b) Fastening at least one first reinforcing ring to the screen cylinder at at least one first land area, to provide stability to the screen cylinder. And, (c) fastening at least one second reinforcing ring to at least some of a plurality of the second land areas in at least one row of grooves, to provide additional stability to the cylinder without significantly adversely impacting the flow of accepts through the slots.
Step (c) may be practiced by welding at least one second reinforcing ring to each of substantially all of the second land areas in a row of grooves. The reinforcing ring may be welded to the land portions between the grooves by directing a laser beam e.g. radially through the ring material. The laser beam is then directed through the outer cylindrical side plane of the ring towards a land portion between two grooves. A hidden weld is formed in the contact area between the inner cylindrical side plane of the ring and the respective land portion.
If the radial extension of the ring is large, that is if the ring has an axial extension, e.g. &gt;5 mm [i.e. too big for the laser beam to penetrate], then the laser beam may be directed from either one of the two radially extending side planes of the ring towards the intended welding spot between the inner cylindrical side plane of the ring and a land portion between two grooves. The laser beam then forms an angle &lt;90E, typically about 30E-50E, with the radius of the ring.
Step (c) may be practiced by looping a completely formed metal ring over the cylinder outer surface for an outflow screen cylinder, or inserting a completely formed ring into the hollow interior (and sliding it down) for an inflow type screen cylinder. Alternatively where the screen cylinder is an outflow screen cylinder step (c) may be further practiced by looping a partially formed ring--having free ends--around the outer surface of the screen cylinder, and fastening the free ends of the partially formed ring together while the ring is traversing the second land areas to which it is to be welded. When the screen cylinder is to be used with rotors having a power consumption that is above about 30 kW/m.sup.2 of cylinder surface area, step (c) may also be practiced by looped a punched cylindrical shell over the rings, or alternatively be practiced by looping the punched cylindrical shell over the cylinder, in this case the "ring" not being solid, but being the punched cylindrical shell.
The invention also relates to a method of using a screen cylinder to screen cellulose pulp from the pulp and paper industry, the screen cylinder as described above. This method comprises the steps of: (a) Causing the cellulose pulp to flow in a primarily circumferential path along the inlet side surface; and while the pulp is flowing in the substantially circumferential path: (b) causing accepts to pass through the slots to the outlet side surface without the flow thereof significantly adversely impacted by the at least one second reinforcing ring; and (c) causing rejects to pass along the inlet side surface to be moved away from engagement with the screen cylinder. Steps (a)-(c) are typically practiced with the pulp at a consistency of between about 0.3-6.0%, preferably between about 0.3-1.5%. Step (a) may be practiced using a rotor. If the rotor has a power consumption that is above about 30 kW/m.sup.2 of cylinder surface area, then the screen cylinder typically further comprises a punched cylinder disposed over, and connected to, the first and second reinforcing rings, providing further reinforcement to the cylinder, and steps (a)-(c) are practiced with pulp at a consistency of between about 1.5-6.0%.
The invention also relates to a screen (such as a pressure screen) for screening pulp. The screen comprises the following components: An inlet for suspension to be screened. An outlet for accepts. An outlet for rejects. A pulsing structure (such as a rotor, especially where the screen cylinder remains stationary); and a screen cylinder, particularly the screen cylinder as specifically described above in which at least one second reinforcing ring is welded to substantially all of the second land areas in at least one row of grooves to provide additional stability to the cylinder, while not significantly adversely impacting the flow of accepts through the slots. And, the screen cylinder is positioned with respect to the outlet so that accepts flow through the slots from the inlet to the accepts outlet, and rejects flow along the inlet surface of the screen cylinder and then ultimately through the rejects outlet.
Each groove formed in a screen cylinder of the present invention may be a groove having a screening slot parallel with the groove, and disposed therein. The slot is preferably disposed substantially in the bottom of the groove, but may be disposed on either of the side planes of the groove. The groove may in some special embodiments be formed by the screening slot itself, if no additional larger relief groove is needed in the screen. The groove may have screening openings of other form than slots disposed therein, such as round holes or oblong openings.
The grooves on the outlet side of the screen cylinder, i.e. the relief grooves, are according to a preferred embodiment of the present invention connected through screening openings, such as slots, to contoured grooves on the inlet side of the screen cylinder, said contoured grooves having an upstream side plane, a bottom and a downstream side plane. The contoured grooves [and the screens utilizing them] may be formed as shown in U.S. Pat. Nos. 4,529,520, 4,836,915, 4,880,540, and/or 5,000,842, the disclosures of which are hereby incorporated by reference herein.
According to another aspect of the present invention a method of manufacturing a screen cylinder is provided. The method comprises the steps of: (a) Constructing a metal cylinder having an outer surface, an inner surface, a central axis, and an effective axial length, one of the inner and outer surfaces comprising an outlet side of the cylinder, and the other of the inner and outer surfaces comprising an inlet side of the cylinder, by: (a1) forming in the outlet surface a plurality of grooves substantially parallel to the central axis; and (a2) forming a slot in at least some of the grooves, each slot defining a through-extending flow path of a predetermined size between the inlet and outlet surfaces. And, (b) fastening at least one metal reinforcing ring to the screen cylinder in a substantially spiral configuration, extending over the grooves on the outlet surface to provide stability to the screen cylinder. Step (b) may be practiced by substantially continuously and automatically welding. The cylinder may have land areas at the ends of the effective axial length thereof, and the method may comprise the further step of tack welding the substantial spiral ring to the land areas. Step (b) may be practiced by rotating the cylinder very slowly while feeding the metal bar as the reinforcing ring into operative association with a continuous welding machine. Step (a) may also be practiced to provide a cylinder with staggered grooves and slots.
It is the primary object of the present invention to provide a screen cylinder, screen using the screen cylinder, method of use of the screen cylinder, and method of manufacture of the screen cylinder, that allow increased capacity of a screen cylinder without significantly adversely affecting screen strength, and/or enhanced accepts cleanliness. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.